XMLBear: Process error correctly

Errors were shown as logs instead of errors. Corrected the processing of
output to show the error messages.

Fixes coala#251

bears/xml/XMLBear.py

@@ -1,3 +1,5 @@
+import collections
+
 from coalib.bearlib.abstractions.Lint import Lint
 from coalib.bears.LocalBear import LocalBear
 from coalib.misc.Shell import escape_path_argument
@@ -24,10 +26,16 @@ class XMLBear(LocalBear, Lint):
     gives_corrected = True
 
     def process_output(self, output, filename, file):
-        if self.stdout_output:  # only yield Result if stdout is not empty
-            return self._process_corrected(self.stdout_output, filename, file)
-        if self.stderr_output:  # pragma: no cover
+        if self.stderr_output:
+            self.use_stderr = True
+            if self.stdout_output:  # only yield Result if stdout is not empty
+                return collections.ChainMap(
+                           self._process_issues(self.stderr_output, filename),
+                           self._process_corrected(self.stdout_output,
+                                                   filename, file))
             return self._process_issues(self.stderr_output, filename)
+        elif self.stdout_output:  # only yield Result if stdout is not empty
+            return self._process_corrected(self.stdout_output, filename, file)
 
     def run(self, filename, file,
             xml_schema: path="",